1. Field of the Invention
The present invention relates generally to transmission of digital information, and more particularly, to a data transmission cable that can be used interchangeably in both SCSI and parallel port applications.
2. Description of the Prior Art
Computer peripherals, such as printers, modems, disk drives, and the like, are often external to a main computer chassis and must be connected to an input/output port of the computer via a data transmission cable. A connector at one end of the cable connects with the input/output port connector on the computer, while the connector at the other end of the cable connects to the input/output port connector on the peripheral device. The physical and electrical characteristics of a data transmission cable depend upon the type of input/output port protocol for which it must carry data. For example, a cable designed to connect a Small Computer Systems Interface (SCSI) peripheral to the SCSI port of a computer will differ physically and electrically from a cable designed to carry data between a printer and a computer's parallel port.
Prior art cables are available for connecting a SCSI peripheral to the SCSI port of a host computer. These cables come in a variety of connector types and sizes. For example, some prior art SCSI cables employ 50-pin Centronics connectors at each end, while others employ 50-pin High Density connectors. Other prior art SCSI cables are designed to connect a SCSI peripheral to the 25-pin DB SCSI connector typically found on Apple.RTM. Macintosh.RTM. computers. These prior art cables typically have a male or female DB25 connector at one end, and a 50-pin Centronics.RTM. connector at the other end.
Still other prior art SCSI cables have DB25 connectors at both ends. These latter cables typically have a main cable body that comprises 19 twisted pairs, each twisted pair having a first conductor and a second conductor. The first conductor in each of the first eighteen twisted pairs is coupled directly to a respective one of the pins at each end of the cable. These eighteen conductors carry data/control signals in accordance with the SCSI protocol. The second conductor in each of these first eighteen twisted pairs defines a "return" for the respective data/control signal carried on the first conductor of that twisted pair. Pin 25 of the cable is reserved for termination power, and both conductors of the 19th twisted pair are connected to pin 25 to provide increased current carrying capacity for the termination power.
Because eighteen pins are used for data/control signals, and pin 25 is reserved for termination power, there are only six remaining pins to handle the eighteen signal "returns", i.e., the second conductor in each of the first eighteen twisted pairs. Prior art SCSI cables typically distribute the eighteen signal "returns" evenly over the six remaining pins. That is, each of the six remaining pins is tied to three of the eighteen signal "return" conductors. Table 1 illustrates the even distribution of "returns" in a prior art SCSI cable. As shown, pins 7, 9, 14, 16, 18 and 24 each share three of the eighteen signal return conductors in the prior art SCSI Cable.
TABLE 1 ______________________________________ Prior Art SCSI Cable PIN # SCSI FUNCTION ______________________________________ 1 REQ 2 MSG 3 I/O 4 RST 5 ACK 6 BSY 7 ACK Return BSY Return ATN Return 8 DBO 9 MSG Return RST Return SEL Return 10 DB3 11 DB5 12 DB6 13 DB7 14 DB0 Return DB1 Return DB2 Return 15 C/D 16 DB3 Return DB5 Return DB4 Return 17 ATN 18 DB6 Return DB7 Return DBP Return 19 SEL 20 DBP 21 DB1 22 DB2 23 DB4 24 REQ Return I/O Return C/D Return 25 TERM PWR ______________________________________
Prior art parallel port cables that employ DB25 connectors at each end typically comprise only twenty-five single conductors connected to respective pins at each end of the cable. Table 2 shows how the 25 pins/conductors of a parallel port cable are defined and provides a comparison of the parallel port pin definitions to those of a prior art SCSI cable.
TABLE 2 ______________________________________ Comparison of Prior Art Parallel Port Cable to Prior Art SCSI Cable. PIN # PP FUNCTION SCSI FUNCTION ______________________________________ 1 nSTROBE REQ 2 D1 MSG 3 D2 I/O 4 D3 RST 5 D4 ACK 6 D5 BSY 7 D6 ACK Return BSY Return ATN Return 8 D7 DB0 9 D8 MSG Return RST Return SEL Return 10 nACK DB3 11 BUSY DB5 12 ERROR DB6 13 SELECT DB7 14 nAFEED DB0 Return DB1 Return DB2 Return 15 nFAULT C/D 16 nINIT DB3 Return DB5 Return DB4 Return 17 nSELECT IN ATN 18 GROUND DB6 Return DB7 Return DBP Return 19 GROUND SEL 20 GROUND DBP 21 GROUND DB1 22 GROUND DB2 23 GROUND DB4 24 GROUND REQ Return I/O Return C/D Return 25 GROUND TERM PWR ______________________________________
Despite the common connector sizes (i.e., DB25 ) in both prior art SCSI and parallel port cables, a 25-pin prior art SCSI cable cannot be used in parallel port applications because the even distribution of "return" conductors over pins 7, 9, 14, 16, 18 and 24 of the prior art SCSI cable will cause signal integrity problems when such a cable is employed in connection with a parallel port. Specifically, because of the way the return conductors are distributed evenly over pins 7, 9, 14, 16, 18 and 24, the first and second conductors of certain twisted pairs of a prior art SCSI cable will both carry data/control signals when used in connection with a parallel port. As a result, crosstalk may occur over certain twisted pairs.
FIG. 1 is a partial schematic diagram of a prior art SCSI cable that illustrates the problems encountered when such a cable is employed in a parallel port application. The prior art SCSI cable has a first DB25 connector 221 at one end, and a second DB25 connector 223 at the other end. The main body of the cable comprises 19 twisted pairs, e.g., 220, 222, 224, 226 and 228. Each twisted pair has a first conductor, denoted by the letter "a", and a second conductor denoted by the letter "b". As shown, the first conductor 220a of one of the twisted pairs 220 is connected to pin 1 of each connector 221, 223, while the second, or "return", conductor 220b is connected to pin 24. Both the first and second conductors 228a, 228b of a nineteenth twisted pair 228 are connected to pin 25 of each connector 221, 223 in order to carry the SCSI termination power.
The first conductors 222a, 224a, 226a of twisted pairs 222, 224 and 226 are connected to pins 5, 6 and 17, respectively. The second, or "return", conductor 222b, 224b and 226b of each twisted pair 222, 224, 226 is connected to pin 7. As shown in Table 1, pins 5, 6 and 17 are used to carry the ACK, BSY and ATN control signals of the SCSI protocol, while pin 7 provides the "return" for each of those signals. Other data and control signals are carried on other twisted pairs (not shown) and the return conductors for each of those other twisted pairs is tied to either pin 9, 14, 16, 18 or 24. For example, as illustrated in both FIG. 1 and Table 1, pin 24 is connected to the "return" conductor for pins 1, 3 and 15.
To illustrate the problem of using such a prior art cable in parallel port applications, consider pins 5, 6 and 17. Pin 5 of the prior art SCSI cable is tied to the first conductor 222a of twisted pair 222 in order to carry the SCSI "ACK" control signal. The second, or "return", conductor 222b of that twisted pair is tied to pin 7. On a parallel port, pins 5 and 7 are both defined to carry data signals, i.e., DB4 and DB6, respectively. Consequently, if the prior art SCSI cable were employed to transmit signals between two parallel ports, the data signals transmitted on pins 5 and 7 would travel down the respective first and second conductors 222a, 222b of the same twisted pair 222. As a result, these signals may interfere with each other causing data integrity problems. Similarly, because pin 7 is also tied to the return conductors for pins 6 and 17, parallel port data transmitted on pin 7 could also interfere with the data and control signals on pins 6 and 17 (i.e., DB5 and nSELECT IN) of a parallel port. Crosstalk on these common "returns" can cause a parallel port peripheral to malfunction.
As the foregoing illustrates, therefore, a prior art SCSI cable cannot be used to connect a peripheral device to a host device via respective parallel ports. Users of parallel port and SCSI equipment must purchase separate cables for each application. Consequently, there is a need for a single cable that can be used in connection with either a parallel port or SCSI port. The present invention satisfies this need.